1. Field of the Invention
This invention relates to a novel method for the production of bismuth (III) oxide. More particularly, this invention relates to a method for enabling minute substantially spherical bismuth (III) oxide particles of a substantially uniform diameter useful as a raw material for optical materials, electronic materials, and superconducting substances to be efficiently and economically produced with a firing temperature about 200.degree. C. lower than generally employed by the conventional method.
2. Prior Art Statement
In recent years, the demand for bismuth (III) oxide as a raw material for optical materials, electronic materials, and superconducting substances has been growing rapidly. As means of producing the bismuth (III) oxide, a method which comprises adding an alkali such as sodium hydroxide to an aqueous solution containing a bismuth salt thereby inducing precipitation of bismuth hydroxide or bismuth oxide hydrate in the aqueous solution, collecting the precipitate from the aqueous solution, and firing the recovered precipitate in the air and a method which comprises directly firing bismuth nitrate have been employed heretofore. These are very popular methods as introduced in "Rikagaku Jiten (Physicochemical Dictionary)" published in Japan.
These methods, however, have the following disadvantage. First, they require a high firing temperature of not lower than about 550.degree. C. They generally produce rodlike bismuth (III) oxide particles lacking uniformity in diameter. They are incapable of producing spherical bismuth (III) oxide particles. FIG. 1 graphically shows the results obtained in a comparative experiment to be described later. This graph shows the change of weight (curve 1) observed during the production of bismuth (III) oxide by the firing of bismuth hydroxide in accordance with the conventional method and a typical result (curve 2) of differential thermal analysis. It is noted from the graph that, during the firing, the product of firing lost weight as the temperature increased to the neighborhood of 550.degree. C. and reached a constant weight near 550.degree. C. This fact implies that bismuth (III) oxide is formed in the neighborhood of 550.degree. C. FIG. 3 is a photomicrograph showing (at 5,000 magnifications) a typical particulate structure of bismuth (III) oxide obtained by the firing of bismuth hydroxide mentioned above. This photograph clearly shows that the bismuth (III) oxide so produced was in the form of rodlike and disklike crystals.
In the production of optical materials, various electronic materials, and superconducting substances, the bismuth (III) oxide is used in the form of sintered masses obtained by firing. When the bismuth (III) oxide is in the form of rodlike and disklike particles lacking uniformity in diameter, the sintering thereof proceeds slowly. In this case, it is difficult to obtain sintered masses of substantially compact texture. Moreover, since the sintering requires, for example, such a high firing temperature as more than 550.degree. C., the production of the sintered masses entails a high energy cost.
A method which produces bismuth (III) oxide from bismuth alkoxide as a raw material has been known to the art [Marsh Gary Barton, Fanelli Anthony Joseph, Armor John Nelson, and Zambri Patrick Michael: European Patent Application No. EP 199,930 (Cl BOIJ2/04), Nov. 05, 1986, U.S. Application Ser. No. 717,931, Mar. 29, 1985]. This method proves to be expensive because the synthesis of the raw material used therefor is complex from the operational point of view.